Miele T494C Dryer Possible Earth Leakage

[Chook]

This Machine has not been used for several years due to RCD tripping. I have decided to determine the cause.
I inspected (ohms check), cleaned & reinstalled the heater bank. After that it ran for @ 90 seconds then stopped. Thereafter It would trip the RCD when powered ON. I removed & reinstalled the heater bank and did some continuity checks between the wring connectors & the chassis (those seemed OK). Started it again on Normal Dry+ selection and it ran then stopped (still powered on) I assumed that was because the drum had no moisture in it. I then selected cool air and it tripped the RCD. I am not 100% sure if I am doing the short circuit versus earth leakage checks properly. The machine has been handled carefully carefully so I am starting to consider this is a control panel circuit issue.
Any advice appreciated , especially (appliance) earth leakage & short circuit checks

Regards,
Chook

 

[Chook]

Additional info, selected Normal mode, the drum rotated in one direction for 1 minute then paused and rotated in the opposite direction for 20 seconds then tripped. I reset the RCD and it continued rotating the paused to reverse direction but at that point it tripped the RCD. I have seen comments that suggest it may be the heater element breaking down as it heats up. It looked physically OK by visual inspection and the resistance check was @ 26 ohms, I don't understand the science of the heating element "breaking down" under load.

 

[Harry Bloomfield]

The 'breaking down', is leakage between either L to E, or N to E, or maybe a combination of both, and the amount of leakage can be quite variable. Your 26 ohms, is the L to N resistance of the element, but what you need to is measure the insulation resistance between L and E, and N and E. It's a low value of that resistance which trips the RCD and is usually measured at a higher voltage and in millions of ohms.

 

[Chook]

Thanks Harry. I have a rudimentary understanding of resistance however, I surmise my measurement of the cold wire coil only confirms it is not broken, where as the relevant resistance is when the coil is supplied with higher voltage . I presume if the resistance breaks down under this voltage the current will increase to the point that it trips the CB. Am I on the correct track? Can testing the element under higher voltage be done in the field?

 

[Harry Bloomfield]

Thanks Harry. I have a rudimentary understanding of resistance however, I surmise my measurement of the cold wire coil only confirms it is not broken, where as the relevant resistance is when the coil is supplied with higher voltage . I presume if the resistance breaks down under this voltage the current will increase to the point that it trips the CB. Am I on the correct track? Can testing the element under higher voltage be done in the field?

You are not understanding it!

The RCD tripping is not a result of the resistance of the element, the 46 Ohms, it's a breakdown of the insulation between the element and earth.

RCD's work by comparing the flow of current between what goes in on the live, and what comes out on the neutral, if there is a difference of more than 30mA, they trip - based on the L (or N) finding an alternative route. In other words, the insulation is breaking down, under the pressure of voltage. Which is why you need a suitable meter, which checks the insulation resistance, at a suitable voltage. Your multimeter may only apply 9v, insulation resistance instruments, can apply 250v, 500v, and even 1000v, to check if the insulation can be broken down.

Having tried to explain the above, if the machine trips the rcd, when only the motor is used - not the element, then I would suspect the motor as the cause. It could just be carbon dust build up around the motor. Have you tried cleaning the dust from the motor? An air line and compressor works best, to blow the carbon dust out.

 

[Chook]

Thanks Harry, I do understand the basic principle of the RCD function (comparison of outflow vs inflow). What I didn't entirely understand was the term "insulation" but now I think the penny has dropped. I was thinking this "insulation" was a technical term to do with the metallurgy of the element but I now I understand it is about the insulation breaking down. I have visually checked the motor and it is very clean, it is not the easiest to access but I will try some compressed air. The other part of the trouble shoot that doesn't seem logical to me is that in drying mode, the drum rotates & reverses then trips (after approx 80 seconds) however, in "timer cool air" mode is selected it trips straight away (wouldnt cool mode bypass the element. This condition points to a motor problem. I don't have a parts diagram but I presume it either has one motor that drives the drum and the blower (or one for each?). Could it be the blower motor does not start immediately in dryer mode? Logic dictates in "timer" mode the blower would start straight away. These assumptions may explain why it runs for a while before tripping in dryer mode but trips immediately in timer mode?

 

[Harry Bloomfield]

Could it be the blower motor does not start immediately in dryer mode? Logic dictates in "timer" mode the blower would start straight away.

You may be correct, but small fan motors tend to synchronous motors, so no brushes, no carbon flung out to short out and trip the RCD.

 

[Honj]

Can you confirm the exact model of the machine, is it a Novo Basic/Midi/Plus or a Novo Super? I'm not an expert on the model but believe there is a service mode that might help pin point if it its the drive motor or not.

 

[Chook]

Hi Honj, it is a Miele T494C

 

[Chook]

You may be correct, but small fan motors tend to synchronous motors, so no brushes, no carbon flung out to short out and trip the RCD.

Thanks Harry, if it didn’t trip on timer cool mode, I would have the heating element Meggar tested.

 

[Chook]

You may be correct, but small fan motors tend to synchronous motors, so no brushes, no carbon flung out to short out and trip the RCD.

Thanks Harry, if it didn’t trip on timer cool mode. I would get the heating element Meggar tested.

 

[Chook]

I see references to “service mode” , is this similar to “Built In Test Equipment” that can generate BITE Codes or something different?

 

[Honj]

Yes that is correct, Miele in my opinion is one of the best manufacturers to provide diagnostic capability via the front panel - usually it goes further than just producing error codes and can also activate functions such as spin the drum for 20 seconds. Unfortunately though, the instructions can vary wildly between machines. Can you check what the model number is on the machine's data plate? There should be a "Mod" which will say T494C and also a "M-Nr" which should be a bunch of digits.

 

[Chook]

Mod T494C Nr. 10 / 31829200

 

[Honj]

There should also be a reference on that label to a M-Nr or sometimes called a Mat-Nr.

The Nr. you have posted above I think is the serial number of the machine.